In sewage stations, septic tanks, wells, etc., it often occur that solid matter or pollutants, such as socks, sanitary pads, paper, etc., clogs the submersible pump that is lowered into the basin of the system. The contaminations are sometimes too big to pass through the pump if the impeller and the impeller seat are located at a fixed distance from each other.
In order to get rid of the clogging matter, it is known to equip centrifugal pumps with means for cutting up the solid matter into smaller pieces and thereafter evacuate the small pieces together with the pumped liquid. However, the cutting up of the solid matter is energy intensive, which is adverse especially since pumps of this kind usually operates for long periods of time. Another conventional way of getting rid of clogging matter is to use an impeller having only one vane, which present one large throughput channel capable of letting through the solid matter. One drawback with this type of pump is that the solid matter often get tangled around the leading edge of the vane. A third attempt, to solved the problem of large solid matter clogging the pump, use a arrangement in which the impeller is at a fixed distance from the impeller seat, e.g. 30-40 mm. A huge drawback is that the pump has a really low efficiency all the time.
A better way of solving the problem of solid matter clogging the pump should be to admit the impeller and the impeller seat to be movable in the axial direction in relation to each other, in order to form a gap. But known pumps comprising this feature uses said gap for other purposes. Furthermore, they only admit a small gap between the impeller and the impeller seat. In EP 1,247,990 is shown a pump, the impeller of which is movable in the axial direction in relation to the impeller seat along the longitudinal direction of the drive shaft. But the movability is strongly limited and the object solved is only to admit operational start in a dry state, e.g. now liquid in the pump. GB 751,908 shows a pump having a manually controlled movability of the impeller in relation to the impeller seat. The object of this construction is to admit a regulation of the efficiency of the pump. U.S. Pat. No. 6,551,058 shows a pump having an impeller which is movable in the axial direction in relation to the drive shaft. The object of the shown construction is to avoid the vanes of the impeller to be damaged if solid matter enters the pump.
More precisely, none of the abovementioned, or other, documents present a solution, or an object, usable for letting through large pieces of solid matter. Even though small pieces of solid matter might pass through the gap that is formed between the lower edge of the impeller and the impeller seat, it is more likely that large pieces of solid matter will get stuck in the narrow gap formed. In a worst case scenario, the impeller might get totally jammed and thus seriously damage the pump. Such an unintentional shutdown is costly, due to expensive, cumbersome and unplanned maintenance work. It is even better if the sol d matter blocks the inlet of the pump than the solid matter gets jammed between the vane of the impeller and the impeller seat. If the inlet is blocked the only effect is that less fluid will get pumped through the pump, but if the impeller is jammed he pump might get damaged.
A closely related patent, EP 1,357,294 directed to the applicant, shows a pump which is exposed for solid matter included in unscreened sewage water. The pump has a groove in the top surface of the impeller seat for transportation of the entire contaminating subject towards the periphery of the pump housing. However, it is strictly described that the impeller shall not be movable in relation to the impeller seat, due to the object of scraping of solid matter from the vane against the edge of the groove.
Furthermore, submergible pumps are used to pump fluid from basins that are hard to get access to for maintenance and the pumps often operate for long periods of time, not infrequently up to 12 hours a day or more. Therefore it is highly desirable to provide a pump having long durability.